In recent years, from the viewpoint of global environment preservation, instead of boiler-type heating systems that perform heating by burning fossil fuels, heat-pump-type air-conditioning apparatuses using air as a heat source have been increasingly introduced also in cold climate areas. A heat-pump-type air-conditioning apparatus can efficiently perform heating by electrically operating a compressor and by taking heat from air.
On the other hand, however, in a heat-pump-type air-conditioning apparatus, as the temperature (outside air temperature) of air outdoors (outside air) or the like gets lower, frost is more likely to be deposited on an outdoor heat exchanger that exchanges heat as an evaporator between outside air and refrigerant. Accordingly, it is necessary to perform defrosting (remove frost) to melt frost deposited on the outdoor heat exchanger. As an example of a defrosting method, there is a method (hereinafter also referred to as a reverse cycle defrosting) for supplying refrigerant discharged from a compressor to the outdoor heat exchanger by reversing the refrigerant flow used for heating. However, since this method is performed in some cases by stopping indoor heating during defrosting, there is a problem that the level of comfort is decreased.
Thus, to enable heating even during defrosting, for example, the following method (hereinafter also referred to as a heating-defrosting simultaneous operation) is proposed (see, for example, Patent Literature 1). In this method, while defrosting a part of the outdoor heat exchanger by, for example, dividing the outdoor heat exchanger, another outdoor heat exchanger serves as an evaporator to receive heat from outside air and to perform heating.
For example, with the technique disclosed in Patent Literature 1, the outdoor heat exchanger is divided into a plurality of parallel heat exchangers, part of refrigerant at high temperature discharged from a compressor is allowed to flow into the parallel heat exchangers alternately, and the parallel heat exchangers are defrosted alternately. Accordingly, the air-conditioning apparatus according to Patent Literature 1 can perform heating continuously as a whole device. At this time, in a parallel heat exchanger that is a defrosting target, defrosting is performed in a state where the refrigerant pressure inside the parallel heat exchanger is equal to a pressure (pressure corresponding to a temperature relatively higher than zero degrees Celsius in the saturation temperature conversion) that is lower than the discharge pressure and higher than the suction pressure of a compressor, and defrosting is performed with a low refrigerant flow rate by using the latent heat of condensation of refrigerant. In addition, the air-conditioning apparatus according to Patent Literature 1 is configured such that the refrigerant that has been used for defrosting flows into the outdoor heat exchanger that is serving as an evaporator. Furthermore, the air-conditioning apparatus according to Patent Literature 1 determines the presence or absence of frost formation on the basis of a decrease in the suction pressure of the compressor to start defrosting.
In addition, as an air-conditioning apparatus of the related art, an air-conditioning apparatus including a bypass that branches refrigerant discharged from a compressor and supplies the refrigerant to an outdoor heat exchanger is also proposed (for example, see Patent Literatures 2 and 3). The air-conditioning apparatus according to Patent Literatures 2 and 3 enables a defrosting operation (a normal-cycle defrosting operation) in which refrigerant discharged from a compressor is supplied to an outdoor heat exchanger through a bypass without changing the refrigerant flow used for heating and a defrosting operation (an inverse-cycle defrosting operation) in which refrigerant discharged from the compressor is supplied to the outdoor heat exchanger by reversing the refrigerant flow used for heating. In the configuration of the air-conditioning apparatus according to Patent Literatures 2 and 3, during both the normal-cycle defrosting operation and the inverse-cycle defrosting operation, the outdoor heat exchanger is not serving as an evaporator, and heating cannot be performed.